Re: [PATCH] PM / sleep: fix use-after-free on async resume

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

On Wednesday, January 22, 2020 12:03:16 AM CET Rafael J. Wysocki wrote:
> On Tuesday, January 21, 2020 5:54:58 PM CET Rafael J. Wysocki wrote:
> > On Tue, Jan 21, 2020 at 2:31 AM Chanho Min <chanho.min@lge.com> wrote:
> > >
> > > Some device can be released during suspend (e.g. usb disconnection).
> > > But, Its child device still use dev->parent's lock in dpm_wait().
> > > It can be ocurred use-after-free as bellows. This is happened during
> > > usb resume in practice.
> > 
> > In that case the resume of the child is going to be carried out after
> > its parent has gone away, which is generally incorrect..
> 
> That isn't really a problem in the case at hand, though, because the memory
> taken up by the parent can only be freed when all of its children have been
> unregistered and all of the class, type, bus, driver etc pointers of the
> children are NULL then, so there won't be a resume callback to execute for
> the child.

Well, not really true, because device_del() doesn't clear dev->bus, for
example, AFAICS, so the resume really needs to be explicitly avoided if
the device has been deleted.

[cut]

> > > --
> > 
> > Something a bit more sophisticated is needed here, let me think about that.
> > 
> 
> I've ended up with the patch below.
> 
> The lock prevents the unregistration of dev from completing, if it is acquired
> before device_pm_remove() in device_del(), and that prevents the parent
> reference from being dropped (at the end of the latter) until the lock is held.
> If the lock is acquired after device_pm_remove() has been called for the
> device, there obviously is no need to wait for the parent.
> 

So something like this should work:

---
 drivers/base/power/main.c |   42 +++++++++++++++++++++++++++++++++++++-----
 1 file changed, 37 insertions(+), 5 deletions(-)

Index: linux-pm/drivers/base/power/main.c
===================================================================
--- linux-pm.orig/drivers/base/power/main.c
+++ linux-pm/drivers/base/power/main.c
@@ -273,10 +273,38 @@ static void dpm_wait_for_suppliers(struc
 	device_links_read_unlock(idx);
 }
 
-static void dpm_wait_for_superior(struct device *dev, bool async)
+static bool dpm_wait_for_superior(struct device *dev, bool async)
 {
-	dpm_wait(dev->parent, async);
+	struct device *parent;
+
+	/*
+	 * If the device and its parent are both resumed asynchronously and the
+	 * parent's callback deletes both the device and the parent itself, the
+	 * parent object may be freed while this function is running, so avoid
+	 * that by reference counting the parent once more unless the device has
+	 * been deleted already.
+	 */
+	mutex_lock(&dpm_list_mtx);
+
+	if (!device_pm_initialized(dev)) {
+		mutex_unlock(&dpm_list_mtx);
+		return false;
+	}
+
+	parent = get_device(dev->parent);
+
+	mutex_unlock(&dpm_list_mtx);
+
+	dpm_wait(parent, async);
+	put_device(parent);
+
 	dpm_wait_for_suppliers(dev, async);
+
+	/*
+	 * If the parent's callback has deleted the device, it is not correct to
+	 * attempt to resume it, so avoid doing that then.
+	 */
+	return device_pm_initialized(dev);
 }
 
 static void dpm_wait_for_consumers(struct device *dev, bool async)
@@ -621,7 +649,8 @@ static int device_resume_noirq(struct de
 	if (!dev->power.is_noirq_suspended)
 		goto Out;
 
-	dpm_wait_for_superior(dev, async);
+	if (!dpm_wait_for_superior(dev, async))
+		goto Out;
 
 	skip_resume = dev_pm_may_skip_resume(dev);
 
@@ -829,7 +858,8 @@ static int device_resume_early(struct de
 	if (!dev->power.is_late_suspended)
 		goto Out;
 
-	dpm_wait_for_superior(dev, async);
+	if (!dpm_wait_for_superior(dev, async))
+		goto Out;
 
 	callback = dpm_subsys_resume_early_cb(dev, state, &info);
 
@@ -944,7 +974,9 @@ static int device_resume(struct device *
 		goto Complete;
 	}
 
-	dpm_wait_for_superior(dev, async);
+	if (!dpm_wait_for_superior(dev, async))
+		goto Complete;
+
 	dpm_watchdog_set(&wd, dev);
 	device_lock(dev);